What serial are you trying to do on the serial? There are two solutions. One would be the one you found and that is currently not supported by resin as we don’t provide yet this dtb (but we will do soon). Another option would be to set core_freq=250 in config.txt (see https://resin.io/blog/updating-config-txt-remotely/ on how to do it). As well you would need to change (manually) the cmdline.txt and make sure that console=tty1 so kernel will not use the serial port. And this should set you up.

This looks like an issue with the default baudrate of the UART port. Which is, if nobody configured otherwise, 9600. You can use stty to change the baudrate as needed. This used to be 115200 on old resin OSs when we were having console argument in the kernel cmdline.